The present disclosure relates to a fixing device fixing a image onto a recording medium, such as a sheet, and an image forming apparatus, such as a copying machine, a printer, a facsimile or a multifunction peripheral, including the fixing device.
In recent years, in an art of an electrographic image forming apparatus, a belt type fixing device rotating an endless fixing belt made of a thin material having a metal layer and a resin layer together with a pressuring roller tends to diffuse. The belt type fixing device can be designed so as to shorten a warming up time by using the fixing belt with a low heat capacity as compared with a conventional heat roller type fixing device.
In the belt type fixing device, inside the fixing belt, a heat source body, e.g. a halogen heater, is provided, and then, the fixing belt is heated by radiant heat emitted from the heat source body. Moreover, a length in an axial direction of the fixing belt is set longer than a maximum width of a sheet passing through a nip part between the fixing belt and the pressuring roller and a length in an axial direction of the heat source body is also set by an equivalent length of this.
In a case where the image forming apparatus carries out a printing operation continuously for a long time, the fixing belt is heated by the heat source body for a long time. In this case, in a passing area as an area in the nip part where the sheet passes through, heat is absorbed by the sheet passing through the nip part. However, in a non-passing area outside the passing area in the axial direction in the nip part, the heat is not absorbed by the sheet. Because of this, the fixing belt may become an excessive temperature rise state at the non-passing area.
In order to prevent the fixing belt from becoming the excessive temperature rise state at the non-passing area, the belt type fixing device covering an end part of the heat source body positioned at the non-passing area by a cover is developed. However, this cover has the following problem. That is, because the cover is merely a nonporous metal plate, radiant heat emitted from the heat source body is reflected at the end part of the heat source body by an inside face of the cover. As a result, it is feared that surface temperature of the end part of the heat source body (e.g. a sealing part of the halogen heater or an end part of a valve) becomes excessive and exceeds a surface limit temperature of the end part of the heat source body.
In the belt type fixing device, in order to control the temperature of the fixing belt, a temperature sensor, for example, using a thermistor, is arranged at an outer circumference side of the fixing bet. A plurality of temperature sensors are provided and positioned, for example, at a center part and an end part in the axial direction of the fixing belt, respectively. Out of these temperature sensors, the temperature sensor positioned at the end part has the following problem. That is, if the fixing belt becomes the excessive temperature rise state at the non-passing area by the continuous printing operation for a long time, it is feared that the temperature of the end part of the fixing belt positioned at the non-passing area exceeds an upper limit of a temperature detectable range of the temperature sensor and the temperature of the end part of the fixing belt cannot be certainly detected.